In digital imaging the maximum CMYK ink coverage can be 400% (4 inks x100%). In printing however that's limited to a significantly lower figure because of the physical caracteristics of the inks and the media you print on. Sheetfed offset printing is generally limited to around 330%, while in other printing techniques sometimes you have to go as low as 240%.

When a designer ignores this technical limitation, the ink that gets laid down last won’t attach properly to the previous layers, leading to muddy browns in shadow areas. The ink also won’t dry properly on the press sheets. Thanks to digital imaging, TAC limits are ruled by color-profiles so in most cases you don't have to deal with it. However there are some cases when you have to re-adjust TAC manually:
When you do not have a specific color profile required to a particular press and media
When you have a cleaned, retouched image and don't want to re-contaminate the plates by changing color-profile
When you have to calculate with additional (spot) colors on top of CMYK inks
The good news is that there is a plugin for Photoshop that helps you to readjust maximum ink in just a few seconds.